Electric power tool

ABSTRACT

Disclosed is an electric power tool having a gear mechanism, which reduces a noise by absorbing collision of gears and is resistible to a high torque. A power transmission key  7  composed of high-strength plastic is provided between an output shaft  3  and a gear  4,  and a plane or a V-groove is formed on each of the output shaft  3  and the gear  4  contacting the power transmission key  7.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a gear mechanism and a torqueresistance mechanism, which reduce a noise of an electric power tool.

[0003] 2. Description of the Related Art

[0004] In a conventional electric power tool, variations in rotationoccurs owing to an excitation force by an alternating current powersupply, a pitch error and an eccentricity of a gear or the like, and asurface of a gear at a drive shaft side and a surface of a gear at anoutput side repeat collision or vibration with each other. In this case,a noise is generated owing to the collision of the surfaces of the gearswith each other, and a collision force of the surfaces of the gears fallinto the excitation force. Therefore, a noise has been generated in aportion other than a gear mechanism unit. Particularly duringactivation, since a torque at the drive shaft side is large, an impactforce thereof is also large. Accordingly, a large noise has beengenerated. In order to reduce the foregoing noise, a precision of thegear has been improved, or a shape of the gear of the surface has beenchanged, thus coping with the noise. In order to reduce a gear noisemechanically, a backlash of a steel key in an inner periphery of thegear, which is for jointing the power transmission shaft and the gear,has been utilized. Note that a configuration using the steel keytransmits a torque by a shear load.

[0005] As a method for improving the precision of the gear, a method ofperforming gear cutting for the gear twice or more, a method ofpolishing the gear surface by use of a cutting sandstone instead of acutter or the like has been performed. However, in such methods, therehave been problems that productivity has been lowered owing to anincrease of the number of processing steps and that a cost has beenincreased accompanied therewith.

[0006] Moreover, even if the precision of the gear is improved, therehas been a problem of deterioration of gearing between the gears owingto an inferior assembly precision with other parts, resulting ingeneration of the noise.

[0007] Furthermore, in the configuration using a space between the steelkey and a key groove, an impact stress applied to the key and the keygroove portion of the gear during the activation has been significantlylarge as compared with that in a configuration in which the gear and theshaft are jointed by forcible insertion, thus causing a problem that ausable range of the configuration has been limited in terms of strength.

SUMMARY OF THE INVENTION

[0008] An object of the present invention is to provide an electricpower tool having a gear mechanism solving the foregoing problems,reducing the noise by absorbing the collision of the gears, and beingresistible to a higher torque.

[0009] The foregoing object is achieved by providing an electric powertool to be described below. Specifically, according to the presentinvention, there is provided an electric power tool, including: a drivedevice; an output shaft transmitting a rotary power of the drive deviceto a gear; a gear disposed on the output shaft with a space therefrom;and a power transmission key composed of an elastic body, the powertransmission key being provided between the output shaft and the gear,wherein a plane or a V-groove is formed on the output shaft and the gearcontacting the power transmission key.

[0010] Moreover, the foregoing object is achieved by providing anelectric power tool to be described below. Specifically, according tothe present invention, there is provided an electric power tool,including: a drive device; an output shaft transmitting a rotary powerof the drive device, the output shaft having at least one of a plane anda V-groove parallel to a center of the shaft; a key composed of plastic,the key having a surface abutting against any one of the plane and aside of the V-groove; and a gear disposed on the output shaft with aspace therefrom, the gear having a groove abutting against the othersurface of the key in an shaft hole portion, wherein power transmissionis performed by use of the output shaft, the key and the gear, and theshaft hole portion of the gear or a member composed of metal contactsthe shaft during application of a large load.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a partially omitted longitudinal section side viewshowing a power transmission unit of a disc grinder according to thepresent invention;

[0012]FIG. 2 is a sectional view showing one example in a firstembodiment of the power transmission unit composed of an output shaftand a gear wheel according to the present invention;

[0013]FIG. 3 is a sectional view showing one example in the firstembodiment of the power transmission unit composed of the output shaftand the gear wheel according to the present invention;

[0014]FIG. 4 is a sectional view showing one example in the firstembodiment of the power transmission unit composed of the output shaftand the gear wheel according to the present invention;

[0015]FIG. 5 is a sectional view showing one example in the firstembodiment of the power transmission unit composed of the output shaftand the gear wheel according to the present invention;

[0016]FIG. 6 is a sectional view showing one example in the firstembodiment of the power transmission unit composed of the output shaftand the gear wheel according to the present invention;

[0017]FIG. 7 is a sectional view showing contact surfaces of an outputshaft and a plastic key;

[0018]FIG. 8 is a sectional view showing a state where a curved surfaceis provided on the contact surface of the plastic key with the outputshaft according to the present invention;

[0019]FIG. 9 is a sectional view showing a state where a slope isprovided on the contact surface of the plastic key with the output shaftaccording to the present invention;

[0020]FIG. 10 is a graph showing deformation amounts of the plastic keysaccording to the present invention;

[0021]FIG. 11 is a graph showing deformation amounts of the plastic keysaccording to the present invention during application of accumulationloads;

[0022]FIG. 12 is a graph showing noise levels caused by an electricpower tool provided with a gear mechanism according to the presentinvention;

[0023]FIG. 13 is a sectional view showing one example in a secondembodiment of the power transmission unit composed of the output shaftand the gear wheel according to the present invention;

[0024]FIG. 14 is a sectional view showing one example in the secondembodiment of the power transmission unit composed of the output shaftand the gear wheel according to the present invention;

[0025]FIG. 15 is a sectional view showing one example in the secondembodiment of the power transmission unit composed of the output shaftand the gear wheel according to the present invention;

[0026]FIG. 16 is a sectional view showing one example in the secondembodiment of the power transmission unit composed of the output shaftand the gear wheel according to the present invention;

[0027]FIG. 17 is a sectional view showing one example in the secondembodiment of the power transmission unit composed of the output shaftand the gear wheel according to the present invention;

[0028]FIG. 18 is a sectional view showing the contact surfaces of theoutput shaft and the plastic key;

[0029]FIG. 19 is a sectional view showing a steady state or anactivation state of the power transmission unit according to the presentinvention;

[0030]FIG. 20 is a sectional view showing a locked state of the powertransmission unit according to the present invention;

[0031]FIG. 21 is a sectional view showing one example where a receiveris directly provided in the gear wheel itself in a modification exampleof the second embodiment;

[0032]FIG. 22 is a sectional view showing one example where the receiveris directly provided in the gear wheel itself in the modificationexample of the second embodiment;

[0033]FIG. 23 is a sectional view showing one example where the receiveris directly provided in the gear wheel itself in the modificationexample of the second embodiment;

[0034]FIG. 24 is a sectional view showing one example where the receiveris directly provided in the gear wheel itself in the modificationexample of the second embodiment; and

[0035]FIG. 25 is a sectional view showing one example where the receiveris directly provided in the gear wheel itself in the modificationexample of the second embodiment.

PREFFERED EMBODIMENTS OF THE INVENTION

[0036] With a disc grinder using a spiral bevel gear shown in FIGS. 1 to12 as an example, a first embodiment will be described. As shown in FIG.1, rotation of the disc grinder is decelerated by a pinion gear 2attached to a rotator 1 of a drive device and a gear wheel 4 attached toan output shaft 3 side. To the output shaft 3 side, a grinding sandstone5 is attached, and configured to grind a steel material or the like bytransmitting a torque from the rotator 1 to the sandstone 5. FIGS. 2 to6 are some examples showing features of the first embodiment. In eachexample, a space 6 of about 0.005 mm to 0.1 mm is provided at an engagedportion between the output shaft 3 and the gear wheel 4 in acircumference direction so that the gear wheel 4 can move with respectto the output shaft 3 in a rotary direction, moreover, a space of about0.1 mm to 0.5 mm is provided between a high-strength plastic key 7 andthe output shaft, furthermore, each of the output shaft 3 and the gearwheel 4 is provided with at least one of a plane and a V-groove parallelto an shaft center, thus the plastic key 7 as a power transmission keyhaving a surface abutting against the foregoing plane or the V-grooveand being low in elastic modulus is provided. Note that a Young'smodulus (Pa=N·m⁻²) of the plastic key 7 is set to a value in a range of3,000 MPa to 30,000 Mpa, and preferably in a range of 3,000 MPa to17,000 Mpa and further preferably in a range of 5,000 MPa to 17,000 Mpa.

[0037] Thus, the output from the pinion gear 2 attached to the rotator 1can be transmitted to the output shaft 3 through the gear wheel 4 andthe plastic key 7. As described above, not a steel key but the plastickey 7 is used, thus making it possible to absorb an impactive torquegenerated during the input by use of elastic deformation thereof.Therefore, the impact can be absorbed. Moreover, the number of plastickeys 7 or the like can be selected appropriately in response to a sizeof a necessary transmission torque, and further, has an attenuationcapability more excellent than that of the steel key. Hence, the plastickey 7 can absorb a vibration and reduce a noise. Furthermore, as ahigh-strength plastic material, among thermoplastic and thermosettingresins, one including a glass fiber or a carbon fiber mixed thereto toenhance strength thereof is desirable. Since the thermoplastic resin ishigh in heat-dependence, sometimes, it cannot maintain a property at anormal temperature in the case where it is used for a mechanical part ofthe electric power tool frequently used at a high temperature.Accordingly, an attention must be paid to a part used at a hightemperature (about 150° C. or more). Moreover, the thermosetting plasticgenerally has a bending elastic modulus higher than the thermoplasticand an attenuation property about one-third that of the thermoplastic.Therefore, with the thermosetting plastic, in some cases, it isdifficult to obtain a noise reduction effect. Taking the above intoconsideration, a material having strength sufficient for meeting a usingcondition must be selected.

[0038] Next, description will be made for key shapes with reference toFIGS. 7 to 9. A structure is adopted, in which the gear wheel 4 isrotated with respect to the output shaft 3 in a circumference direction.Hence, as shown in FIG. 7, in the case where a contact surface 7 a ofthe plastic key 7 with the output shaft 3 is made flat, in an abuttingportion of the contact surface 7 a of the plastic key 7, which contactsan abutting surface edge portion 3 a of the output shaft 3, acompression stress larger than in the other portions occurs, resultingin large deformation. Thus, as shown in FIGS. 8 and 9, the plastic key 7is configured so that a load from the gear wheel 4 can be applied to thesurface of the output shaft 3 continuously and evenly by providing acurvature or a slope for the contact surface 7 a thereof. Thus,durability of the plastic key 7 can be improved.

[0039] Next, in FIG. 10, results of comparison for the deformationamounts of the plastic key 7 are shown. In this case, the output shaft3, the gear wheel 4 and the plastic key 7 were disposed as in FIG. 2,the gear wheel 4 was fixed, and the output shaft 3 was rotated. Theatmospheric temperature was set at 30° C., and for the plastic material,simple polyamide resin or polyamide resin added with a glass fiber wasused. Moreover, the contact surface 7 a of the plastic key 7, whichcontacted the output shaft 3, was made flat. An abscissa of FIG. 10represents the deformation amounts of the plastic key 7, and an ordinatethereof represents the torques applied to the output shaft 3. Theplastic key having a bending elastic modulus of 3000 MPa at a normaltemperature indicates a deformation amount of 0.9 mm with a torque ofabout 20 N·m. Thereafter, even in the case where a rotary angle of theoutput shaft 3 is increased, the torque was not increased, and theplastic key 7 was plastically deformed in the axis direction of theoutput shaft 3, and was not recovered. Originally, for the purpose ofabsorbing the impact load, the bending elastic modulus is desirablysmall. However, a material having strength sufficient for coping withthe load must be selected.

[0040] Next, FIG. 11 shows the deformation amount of each plastic key 7when the output shaft 3 is applied with torques certain times. Theatmospheric temperature was set at 80° C. Moreover, loading conditionswere set so as to accumulate the torques actually applied thereto.Condition 1 was set so as to load a torque of 20 N·m 200000 timesrepeatedly. Condition 2 was set, in addition to the condition 1, so asto load a torque of 18 N·m 120000 times. Condition 3 was set, inaddition to the condition 2, so as to load a torque of 15 N·m 300000times. Condition 4 was set, in addition to the condition 3, so as toload a torque of 12 N·m 140000 times. Although the following depends ona type of the electric power tool, in the case where the tool is a discgrinder or the like, which rotates at a high speed, as the deformationamount is increased, a tendency is shown, in which the backlash of thetool in the circumference direction is increased, and the noise isincreased. Accordingly, it is desirable that the deformation amount ofthe plastic key 7 be as small as possible. From the measurement results,it is understood that, under any condition, the plastic key 7 having alarge bending elastic modulus of 12000 MPa has the least deformationamount among the plastic keys 7, each having the contact surface 7 amade flat. For comparison with this, the same test was performed for theplastic key 7, of which surface shape is made to have a curvature asshown in FIG. 8, the surface contacting the output shaft 3. As a result,in comparison with the one, in which the contact surface 7 a of theplastic key 7 was made flat, the deformation amount was reduced to about⅓. The reason is that, in the case where the contact surface 7 a of theplastic key 7 is made flat, when a certain rotary angle occurs in theoutput shaft 3, a large compression stress is locally generated in thecontact portion of the abutting surface edge portion 3 a of the outputshaft 3 and the plastic key 7, thus the concerned portion is plasticallydeformed. On the contrary, the plastic key 7 having a curvature on thecontact surface 7 a evenly contacts the output shaft 3 when the outputshaft 3 is rotated. Hence, the surface pressure is reduced, resulting inthe reduction of the deformation amount. Thus, the durability of theplastic key can be improved by setting the curvature or the slope inaccordance with the shape of the output shaft 3.

[0041] Next, FIG. 12 shows measurement results for the noises of theelectric power tool on which the plastic key made of polyamide of 12000MPa and having a curvature is mounted. Here, description will be madealso for the measurement results for the noise of the electric powertool in which the gear wheel 4 is forcibly inserted to the output shaft3. Note that the results are for the gear noises including motor noises,which are activation noises during the input and the steady noises at acertain revolution number. In comparison with the electric power tool onwhich the forcibly inserted gear is mounted, in the electric power toolin this embodiment, the activation noise was reduced by 5 dB, and thesteady noise was reduced by 2 dB. Here, the noise reduction was achievedby suppressing the noise thanks to an attenuation effect during thedeformation of the plastic key 7, which was brought by the compressiongiven thereto, when the impact torque from the gear wheel 4 with thecollision of the gear surfaces of the gears was transmitted to theoutput shaft 3.

[0042] Next, description will be made for a second embodiment withreference to FIGS. 13 to 17. FIGS. 13 to 17 are some examples showingfeatures of the second embodiment. Each configuration of the examplesincludes: the shaft 3 having at least one of a plane and a V-groove; andthe plastic key 7 abutting against the shaft 3. The number of plastickeys 7 or the like can be appropriately selected in accordance with asize of the necessary transmission torque. Note that, when an impactivelarge torque is applied to the gear wheel 4, a shear stress is generatedin the plastic key 7, leading to a possibility of breaking the plastickey 7. Therefore, as shown in the drawings, shapes 8 and parallel to thecontact surface 7 a of the plastic key 7 with the output shaft 3 areprovided in a shaft hole portion of the gear wheel 4. In order to formthe shapes 8 and 8 a integrally with the gear wheel 4, processing suchas casting, sintering and wire cutting, alternatively insertion of ametal member made of a material having strength larger than the plastickey 7 can be adopted. Furthermore, a height of each shape is required tobe set lower than that of the plastic key 7.

[0043] Next, description will be made for the second embodiment togetherwith one example where the power is transmitted only by the plastic keyas shown in FIGS. 18 to 20. From the gear wheel 4, the torque is appliedto the plastic key 7 and the output shaft 3 by a mechanism fortransmitting the torque. Thus, a force rotating the output shaft 3 in arotary direction reverse to the rotary direction of the gear wheel 4 isapplied thereto. Therefore, the plastic key 7 is compressed. When thetorque is small, the plastic key 7 is only subjected to the compresseddeformation, and when the torque is released, the plastic key 7 isrecovered. However, when a particular impactive large torque is appliedthereto, the deformation amounts of the plastic key 7 exceeds an elasticdeformation range. In this case, only by the plastic key, as shown inFIG. 18, a crack 9 occurs due to the shear stress. Accordingly, theplastic material having a size mountable onto a small-size gear of suchas an electric power tool has some limitations in a range where it isused as a key even if the plastic material is highly strong and high inheat resistance. In this connection, a configuration is adopted suchthat, during the steady rotation or the activation shown in FIG. 19, theoutput shaft 3 contacts only the plastic key 7 by rotating the gearwheel 4, and that the power transmission is made therebetween. And whena larger torque is applied to the plastic key 7 as shown in FIG. 20, theplastic key 7 is pressed by the output shaft 3, and the output shaft 3contacts a step difference portion 8 a of the gear wheel 4. Even if afurther larger torque is applied, most of the loads from the gear wheel4 are transmitted to the output shaft 3 through the metal member 8composed of the other metal as shown in FIGS. 13 to 17. And when a loadreceiver is formed integrally with the gear wheel 4 as shown in FIGS. 21to 25, the torque is transmitted to the output shaft 3 through the loadreceiver.

[0044] Note that, since the impact torque is not always applied, thedeformation of the plastic key 7 is recovered by reducing the torque astime passes, and the torque transmission is performed through theplastic key 7 again. In this configuration, a position where the outputshaft 3 contacts the step portion of the shaft hole of the gear wheel 4,and a step difference amount thereof with the plastic key must beappropriately set. Moreover, in this embodiment, a material havingstrength larger than that of the plastic key 7 is used for the membercontacting the shaft during the application of the large load. This isbecause, only with the foregoing plastic key 7, a large shear stress isgenerated due to the contact with the output shaft 3, causing breakageof the plastic key 7. As the material, an iron-series material isdesirable. Furthermore, it is desirable that, for a type of the toolwhich is frequently used during the application of the large load, theiron series material be subjected to a heat treatment, thus enhancingthe shear resistance and abrasion resistance.

[0045] Therefore, according to the configuration of the foregoing secondembodiment, the collision force between the surface of the gear at thedrive side and the surface of the gear at the output side is absorbed,the noise caused by the collision can be reduced, and the breakage ofthe plastic key is prevented by eliminating the application of the loadto the key to a certain extent. Thus, the application of the plastic keyto the type of the electric power tool, to which a heavy load isapplied, is enabled.

[0046] According to the present invention, the collision force betweenthe gear surfaces in the gear mechanism unit is absorbed, thus it ispossible to provide an electric power tool having a gear mechanismresistible to a large torque, in which noise reduction is achieved.

What is claimed is:
 1. An electric power tool, comprising: a drivedevice; an output shaft transmitting a rotary power of said drivedevice; a gear disposed on the output shaft with a space therefrom; anda power transmission key composed of an elastic body and providedbetween said output shaft and said gear, wherein any one of a plane anda V-groove is formed on each of said output shaft and said gearcontacting said power transmission key.
 2. The electric power toolaccording to claim 1, wherein said power transmission key is made ofhigh-strength plastic.
 3. The electric power tool according to claim 1,wherein any one of a curvature and a slope is provided on an abuttingsurface of said output shaft and said power transmission key.
 4. Anelectric power tool, comprising: a drive device; an output shafttransmitting a rotary power of the drive device, the output shaft havingat least one plane and a V-groove parallel to a center of the shaft; akey composed of plastic, the key having a surface abutting against anyone of the plane and a side of the V-groove; and a gear disposed on theoutput shaft with a space therefrom, the gear having a groove abuttingagainst the other surface of the key in a shaft hole portion, whereinpower transmission is performed by said output shaft, said key and saidgear, and said shaft contacts any one of the shaft hole portion of saidgear and a member composed of metal during application of a large load.5. The electric power tool according to claim 4, wherein the membercontacting the shaft during the application of the large load is formedof a material having strength exceeding that of said plastic key.
 6. Theelectric power tool according to claim 4, wherein the member composed ofmetal is inserted to the shaft hole portion.
 7. The electric power toolaccording to claim 4, wherein said member composed of metal is formedintegrally with said gear.